Loop-antenna tuning system



Patented May 3, 1949 LOOP-ANTENNA TUNING SYSTEM Bernard D. Loughlin, Lynbrook, N. Y., aslignor to Hazeltine Research, Inc., Chicago, 111., a corporation of Illinois Application December 17, 1946, Serial No. 716,855

7 Claims. 1

The present invention relates to loop-antenna tuning systems and, particularly, to such systems in which a loop-antenna circuit is tuned to any selected operating frequency within a wide range of such frequencies.

In recent years, it has become the practice to utilize loop antennas rather extensively with broadcast receivers. The loop antennas heretofore used frequently have been of the highimpedance type in which the loop-antenna inductance constitutes the entire inductance of a first radio-frequency tuned circuit of the receiver. In many cases, however, loop antennas of the low-impedance type, in which the loop-antenna inductance is generally a small part of the total inductance of the first radio-frequency tuned circuit, can yield superior performance and may result in a more economical design.

As will presently be explained more fully, the problems presented in connection with the tuning of antenna systems utilizing high-impedance loop antennas are quite different from those encountered in antenna systems employing lowimpedance loop antennas. Many of these problems are largely avoided in the loop-antenna tuning system of the present invention which is adapted to use either a high-impedance or lowimpedance loop antenna as desired. To understand the matter more fully, the relative merits of the high-impedance and low-impedance loop antennas will first be briefly considered following which the limitations and disadvantages inherent in prior loop-antenna tuning systems utilizing such antennas will be discussed.

There are several factors influencing the choice of a high-impedance or low-impedance loop antenna in a particular application. In free space, a high-impedance loop antenna usually will provide a performance superior to a low-impedance loop antenna having the same volume. term "volume is meant the product of the crosssectional area and depth of the antenna; it is the cubic content of space enclosed by the antenna. The efiectlve volume of a loop antenna helically wound with its conductor turns in a plane, commonly called a pancake loop antenna, is that of an equivalent solenoid type of antenna in which each conductor turn has the same crosssectional area as any other conductor turn and the several turns are wound in longitudinal depth.

In practice, however, loop antennas can seldom be located in the equivalent of free space and, further, high-impedance loop antennas can rarely be constructed fully to utilize the available volume. A low-impedance loop antenna may thus be preferable where the effective area of a lowimpedance loop can be appreciably greater than the efiective area of a high-impedance loop. The efiective area of a solenoid type of loop is its cross-sectional area; that of a pancake loop is are difficult to control to the required degree and By the o the distributed capacitance may be too great to permit the loop-antenna system to be tuned over the desired band of wave-signal frequencies in the absence of a special and perhaps costly type of tuning condenser.

The low-impedance type of loop antenna may also be desirable in those applications where the effective Q, or ratio of inductive reactance to resistance, of the high-impedance loop is seriously impaired by its close proximity to the metallic chassis of the receiver or other metallic objects or seriously changes when the loop is moved with respect to such chassis during operation, both of which conditions are frequently encountered in the case of portable receivers. Further, and equally important, movement of the loop antenna with respect to the chassis as last mentioned in general will cause detuning of the loop-antenna system to an extent usually much more serious in the case of the high-impedance loop antenna than with the low-impedance loop antenna. Lowimpedance loop antenna thus provide lesser variations of Q and detuning where the loop antenna is normally movably positioned with respect to a metallic chassis or other metallic object.

The low-impedance loop antenna for certain applications may have additional advantages over the high-impedance type of loop antenna. Thus, the low-impedance loop is less susceptible to electrostatic pickup than is the high-impedance loop and consequently will produce less noise disturbances in the receiver when operated in a region of appreciable electrostatic disturbances. Additionally, the low-impedance loop antenna is generally more directional than the high-impedance loop antenna due to the lower electrostatic pickup of the former. A low-impedance loop antenna also has the advantage that it usually facilitates receiver alignment and servicing because it can be fastened directly around the inside of the receiver cabinet and will not interfere with acess to the receiver tubes, trimmer condensers, and the like, and also because it does not require a fixed position with respect to the receiver chassis in order to maintain tracking of those receiver tuned circuits which are tunable by a unicontrol mechanism.

The several enumerated advantages of the lowimpedance loop antenna are offset to a great extent by the difiiculty of attaining with it either or both a reasonably high figure of merit or a uniform figure of merit over a desired tuning band. The term figure of merit" is conventionally defined as the ratio of voltage developed across the condenser of a parallel-resonant circuit coupled to or comprised by the loop antenna to the field strength of a received wave signal expressed in volts per meter. It is a measure of the effectiveness with which a wave signal of given field strength is picked up by the antenna and applied to the first amplifier tube of the receiver.

A high-impedance loop-antenna system is usually tuned over a desired wave-signal band in a rather simple manner by the use of a variable condenser coupled across the terminals of the loop antenna to provide therewith a parallel-resonant circuit constituting the input resonant circult of the receiver. At resonance, the voltage appearing across the tuning condenser is equal to Qe, where Q is related to the resonant circuit comprised by the loop antenna and its tuning condenser, and is usually that of the loop antenna itself, and e is the wave-signal voltage induced in the loop by a received wave signal. Since, however, for a given value of field strength of the received wave signal and a constant value of Q the voltage induced in the loop varies directly with the frequency of the wave signal, the figure of merit of the loop-antenna system is proportional to the frequency of the received wave signal. This is undesirable for many applications for the reason that the signal-to-noise ratio of a receiver operating to receive relatively distant stations varies directly with the figure of merit of the antenna system and thus may be low at the low-frequency end of the tuning band and much higher at the high-frequency end of the band. It is consequently usually desirable that the figure of merit of a loop-antenna system be high and remain substantially constant over the desired tuned range, which is conventionally of the order of approximately three to one, in order to maintain a uniformly high signal-to-noise ratio. In practice the Q of a high-impedance loop-antenna system of the type described may be made to decrease at the high-frequency end of the tuning band to cause the figure of merit to remain more nearly constant over the tuning range. This has the disadvantage, however, that it is usually possible at best only to decrease the Q at the high-frequency end of the tuning band with the result that the figure of merit is usually reduced over most of the band. There is the further disadvantage that reducing the Q as described impairs the selectivity undesirably over an appreciable portion of the tuning band and causes the image ratio of a superheterodyne receiver to be impaired, the image ratio being defined as the ratio with which a desired wave signal is received on tune to that of a wave signal having an image frequency. Lastly, the figure of merit of a highimpedance loop-antenna system may undesirably vary simply by change of the loop Q caused by movement of the loop relative to the receiver chassis or other metal object as earlier mentioned.

A low-impedance loop antenna, on the other hand, frequently is utilized in a loop-antenna system which is tuned by adjustment of a relatively large inductor. The latter is included in series with the loop antenna and with a small fixed condenser to provide a resonant circuit usually comprising the input resonant circuit of the receiver. The wave-signal voltage developed across the condenser is again equal to Qe as in the arrangement first described. Since the induced voltage e varies with frequency as mentioned above, the

figure of merit of this system also is proportional to the wave-signal frequency for a given value of Q and a given field strength of the received wave signal. If the high-impedance and low-impedance loop-antenna systems described are to tune over the same tuning range, it is apparent that the impedance of the low-impedance loop antenna must be smaller than that of the highimpedance loop antenna by the value of the series tuning inductor. Of the composite inductance provided by the low-impedance loop antenna and the tuning inductor, the amount of inductance provided by the loop antenna depends upon the magnitude of the change of inductance which can be obtained by adjustment of the tuned inductor. The greater the change of inductance in the latter, or in other words, the greater the excess tuning range provided thereby, the greater can be the loop inductance and the higher will be the figure of merit of the antenna system.

The relative figures of merit of the high-impedance and low-impedance antenna systems described can be directly compared. If the capacitance of the low-impedance loop system is equal to the minimum capacitance of the high-imped ance loop system, if both loop antennas have the same effective area and shape, and if both antenna systems have the same net value of resonant-circuit Q, the reduction D of the figure of merit of the low-impedance system as compared to the figure of merit of the high-impedance loopantenna system at a given wave-signal frequency is given by the relation:

where Mr is the ratio of maximum to minimum inductance required to tune the system over a desired wave-signal band If the same net value of resonant-circuit Q is assumed for the high-impedance and low-impedance loop-antenna systems, it will be apparent from Equation 1 that the low-impedance system utilizing the series tuning inductor has a uniformly lower figure of merit than does the highimpedance loop-antenna system over the tuning band. This variation of the figure of merit over the tuning band is undesirable, as earlier mentioned, and the impaired figure of merit of the low-impedance antenna system as compared to the high-impedance system works a substantial disadvantage favorin the latter system. Even if the Q of the low-impedance system were made to vary to improve the uniformity of its figure of merit over the tuning band, the Q would have to vary in the order of three to one across the band to maintain the figure of merit substantially constant and the average value of Q may then be rather low with a resultant low value of the figure of merit and greatly impaired selectivity of the antenna system.

To avoid the disadvantages mentioned of the antenna systems above described, it has been proposed that the figure of merit of a loop-antenna system be maintained substantially constant over the tuning band by the use of a lowimpedance loop antenna with a series tuning inductor but including an adjustable inductor coupled directly across the terminals of the loop antenna and adjusted simultaneously with the adjustment of the series inductor. If this ariangement is assumed to have the same value of net resonant-circuit Q as in the low-impedance loop-antenna system last described and both are assumed to have constant Q across the tuning band and to have the same values of M1 and ML, the figure of merit of the last-mentioned proposed arrangement remains substantially constant over the tuning band and is approximately equal to the figure of merit of the earlier described low-impedance system at the geometric mean frequency of the band. In practice, however, the figure of merit of the last-mentioned proposed arrangement is likely to be not higher than the lowest value of the figure of merit of the high-impedance loop-antenna system first described. Such a low value of figure of merit is undesirable from the standpoint that it results in an antenna system having a substantially lower signal-to-noise ratio than can be tolerated in many applications.

It is an object of the present invention, therefore, to provide a new and improved loop-antenna tuning system which avoids one or more of the disadvantages and limitations of prior such systems.

It is a further object of the invention to provide a loop-antenna tuning system which is characterized not only by a substantially constant figure of merit with tuning of the system over a relatively wide range of operating frequencies, but also one having a value of figure of merit substantially larger than heretofore readily obtainable in a. system having a constant figure of merit.

It is an additional object of the invention to provide a new and improved loop-antenna tuning system which may utilize either a high-impedance or low-impedance loop antenna as desired.

In accordance with a particular form of the invention, a loop-antenna tuning system comprises: an inductor; a loop antenna effectively coupled across at least a portion of the inductor to provide therewith a composite value of inductance; and a condenser, including a capacitanceadjusting element, coupled across the inductor to provide a parallel-resonant circuit having a predetermined Q at a frequency within a range of operating frequencies. The system also includes an inductance-adjusting element, associated with the inductor and mechanically coupled to the capacitance-adjusting element for movement in unison therewith to tune the aforementioned circuit over the operating range, having a configuration and movement so related to the configuration and movement of the capacitanceadjusting element as to maintain over the operating range a ratio of composite inductance to capacitance for the circuit varying in opposite sense to any variation of Q thereof from the aforementioned value Q to maintain the figure of merit of the system substantially constant with tuning of the system.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

Referring now to the drawing, Fig. 1 is a circuit diagram, partly schematic, of a complete wave-signal receiver which includes a loop-antenna system embodying the present invention; Fig. la represents the equivalent circuit diagram of the Fig. 1 antenna system and is utilized as an aid in explaining its operation; Fig. 2 is a circuit diagram representing a modified form of the invention; Fig. 3 illustrates, partly schematically, the construction of a loop-antenna tuning system of the Fig. 2 type; and Fig. 4 illustrates a modified form of construction similar to that of Fig. 3.

Referring now more particularly to Fig. 1 of the drawing, the loop-antenna tuning system there shown comprises an inductor l0 and includes a loop antenna il coupled across the inductor III to provide therewith a composite value of inductance. As will presently be described more fully, the inductor ID has its inductance varied to aid in tuning the antenna system over a range of operating frequencies. Since the inductance of the antenna II is effectively coupled in parallel with that of inductor i0 and since only the inductance of the latter is varied for tuning purposes, the coupling of the loop antenna to the inductor It! reduces the range over which the composite inductance of the antenna and inductor may be varied. For a three-to-one range of operating frequencies and assuming that the Q or ratio of inductive reactance to resistance of the system remains constant over the tuning range, it is desirable in accordance with the present invention to attain a three-tc-one range of variation of the composite inductance. This may be accomplished by so constructin the inductor I as to attain as large an inductance variation as possible and by then selecting for the loop antenna ll an effective value of inductance which "with the inductor [0 provides a resultant threeto-one inductance variation. Since the value of loop inductance required to do this is usually rather high, a loop antenna constructed to provide that value of inductance would be unduly expensive because of the requirements that the distributed capacitance of the loop preferably be kept low and its Q be kept high. Consequently, it is more desirable in a practical application of the invention to construct the loop antenna ii to have a medium value of inductance and then to couple it to the inductor iii in a manner hereinafter described in connection with a modified form of the invention in which the value of loop inductance is transformed effectively to the much higher value desired.

The antenna tuning system also includes a condenser l3, having a capacitance-adjusting element, coupled across the inductor l0 to provide a parallel-resonant circuit having a predetermined Q at a frequency within a range of operating frequencies. Preferably, the value of Q of the parallel-resonant circuit i0, I3 remains substantially constant at any frequency within the range of operating frequencies.

The tuning system also includes an inductanceadjusting element i4 associated with the inductor I0 and mechanically coupled to the capacitanceadjusting element of the condenser i 3, as indicated in broken lines, for movement in unison therewith to tune the parallel-resonant circuit l0, l3 over the aforementioned range of operating frequencies. The inductance-adjusting element I4 has a configuration and movement so related to the configuration and movement of the capacitance-adjusting element of the condenser l3 as to maintain over the aforementioned range of operating frequencies a ratio of composite inductance to capacitance for the parallel-resonant circuit varying in opposite sense to any variation of Q thereof from the aforementioned predetermined value of Q to maintain the figure of merit of the loop-antenna tuning system substantially constant with tuning of the circuit i0, i3. Where the Q of the parallelresonant circuit is constant over the tuning range, the ratio of composite inductance to capacitance is then approximately constant. In practice, however, it may be difiicult to maintain constant Q with tuning in which event it is preferable that the ratio of composite inductance to capacitance be made to vary inversely proportional to the square of the variations of Q. The inductame-adjusting element ll comprises a magnetic member, for example a powdered-iron core, movably positioned in the magnetic field of the inductor l0. To obtain a large variation of inductance, and incidentally to retain a substantially constant Q, of the parallel-resonant circuit i0, i3 over the tuning range, the inductor i preferably has an effective length much greater than its diameter. The element [4 then has a length comparable to that of the inductor ID to effect a reasonably linear adjustment of the inductance thereof with movement of the element and thus a composite inductance for the inductor i0 and loop antenna which tends to have an approximately linear inductance variation. Assuming a constant value of Q with tuning, the capacitanceadjusting element of the condenser i3 correspondingly has a configuration to provide a linear adjustment of the capacitance of the condenser with movement of the latter element. Movement of the inductance-adjusting element i4 and the capacitance-adjusting element last mentioned then efiects simultaneous and substantially linear adjustments of the composite inductance of the inductors i0 and ii and the capacitance of the condenser i3 to maintain over the tuning range a substantially constant ratio of composite inductance to capacitance as is desirable for constant Q.

The loop-antenna tunin system above described is coupled to the input circuit of a wavesignal receiver l5, usually being coupled directly to the input electrodes of the first vacuum tube employed in the receiver.

Considering now the operation of the loopantenna tuning system just described, wavesignal energy received by the loop antenna ii is applied to the parallel-resonant circuit i0, ii. The latter is tuned to resonance, by simultaneous adjustments of the inductance-adjusting element i4 and the capacitance-adjusting element of the condenser IS, with the received wave-signal energy, and the Wave-signal potentials developed across the circuit are applied to the input circuit of the receiver l5 for utilization thereby in a conventional manner.

The operation of the antenna system will now further be considered with reference to Fig. 1a which represents the equivalent circuit of the loop-antenna tuning system. In this equivalent circuit, the inductor La. represents the equivalent parallel inductance of the loop antenna II and the inductor l0 and is given by the relation:

where Lb=the inductance of the loop antenna ii; and Lc=the inductance of the inductor iii.

e m'r. 3)

The voltage Ec developed across the condenser I! by the equivalent voltage e is given by the relation:

Ec=Qc (4) where Q=the value of the net resonant-circuit Q which is usually the composite Q of the loop antenna ii and inductor H) in parallel.

From Equations (2) and (3), the equivalent voltage 6 is thus given by the relation:

*Lwt. E) LT (5) However, the induced voltage i oi the loopantenna system is also given by the relation:

(6) where K=an arbitrary constant o=the angular frequency of the received wave signal E=the field strength of the received wave signal.

From Equations 7 and 8, the figure of merit of the antenna system may be expressed by the relation: L K Le (Ll C (9) It will be apparent from Equation 9 that for a constant value of the figure of merit, the ratio of the composite inductance to the capacitance of the loop-antenna system must vary inversely proportional to the square of any variations of the Qthereof; that is,

It is evident from the last-mentioned relationship that the ratio of composite inductance to capacitance of the antenna system should remain constant with tuning if the value of the net resonant-circuit Q of the system is to remain constant over the tuning band. In practice, the Q of the system usually will vary somewhat over the tuning band, in which case the ratio of composite inductance to capacitance of the system should vary in opposite sense with variations of Q to maintain the figure of merit substantially constant and particularly the ratio mentioned should vary as the square of such variations of Q. The ratio of composite inductance to capacitance may be caused to vary as last described by suitable choice of the configuration and related movements of the inductance-adjusting element I4 and the capacitance-adjusting element of the condenser i3. Ordinarily this is not difficult and may perhaps be most readily accomplished by suitably shaping the plates of the condenser II to provide a desired mode of variation of the capacitance thereof.

Fig. 2 is a circuit diagram representing an additionally modified form of the invention essentially similar to that of Fig. 1, similar elements being designated by similar reference numerals and analogous elements by similar reference numerals primed, except that the loop antenna ll.

of the present arrangement is constructed to have a relatively low impedance and is coupled only across a portion of the inductor l. As thus arranged, the inductor it operates in the nature of an auto-transformer so that the inductance of the loop antenna II is effectively increased to the desired value by suitable selection of the relative value of the inductor portions a and b. As will presently be described in more detail, the inductor ID of the present arrangement preferably has the inductor portion awound over the inductor portion b, the two inductor portions being connected to have aiding magnetic fields. This construction of the inductor Ill ensures that both inductor portions are tightly coupled for all positions of movement of the inductance-adjusting element I4. Optimum system performance is obtained by so selecting the relative inductances of the loop antenna ii and inductor it that the portions a and b of the latter are approximately equal. The present arrangement has the important advantage, in addition to its simplicity and the fact that it involves a relatively inexpensive construction, that the wave-signal currents flowing in the circuit of the antenna ii are in opposing phase to those flowing in the parallel-resonant circuit I0, I 3 so that very little wave-signal current flows through the portion a of the inductor Hi. This permits the inductor portion a to be constructed of relatively fine wire where desired, thereby to permit the inductor portion b to be constructed of heavier wire with consequent improvement of the Q of the inductor i0 and of the parallel-resonant circuit in which it is used. The operation of this modified form of the invention is otherwise essentially similar to that hereinbefore described and will not be repeated.

Fig. 3 illustrates, partly schematically, a suitable form of construction of the inductor i0 and condenser l3 of the Fig. 2 arrangement, elements in Fig. 3 corresponding to similar elements of Fig. 2 being designated by similar reference numerals. The inductor ill is, wound upon a suitable cylindrical form ii of insulating material, the inductor portion a being wound with relatively fine wire I! over the inductor portion b which is wound with much heavier wire l8. Preferably the portion a is wound over a separate winding form, not here shown for simplicity, which is then positioned in telescoped relation to the portion b wound on the form iii. The length of the inductor I0 is much greater than its diameter, and the inductance-adjusting element i4 is constructed as a solid cylinder of magnetic material, such as powdered iron, of length comparable to that of the inductor and of a diameter to permit the element to be easily moved within the coil form l6. This construction provides a substantial variation of inductance with movement of the element N. The condenser l3 comprises two stationary condenser plates I9, 20 and a movable plate 2i with dielectric material 22 inserted therebetween. This condenser construction, as is well known, is one which may if desired be arranged to provide a linear variation of capacitance with movement of the condenser plate 2! or may easily be arranged to provide any other desired manner of capacitance variation.

The movable inductance-adjusting element l4 and the movable condenser plate 2| are mecharrically connected for unicontrol adjustment by a member 23.

Fig. 4 illustrates a modified form of construction suitable for the tunable elements of a loopantenna tuning system embodying the present invention. Elements of Fig. 4 corresponding to similar elements of Fig. 3 are designated by similar reference numerals and analogous elements by reference numerals primed. The condenser l3 includes cylindrical stationary condenser plates i9, 20 supported in concentric relation and electrically connected together at one end by a conductive disc 24. The condenser plate 2 I' also is of cylindrical configuration and is arranged to move into telescoping relation with the stationary condenser plates I9, 20'. The condenser plate 2| is closed at one end by a conductive disc 25 and the inductance-adjusting element It is supported by an insulating member 28 from the disc 25 and in coaxial relation therewith. As thus supported, the element It moves with the condenser plate 2| into telescoping relation with the inductor I0 coaxially supported by its coil form i5 from the disc 24. The present construction provides a substantial, and if desired linear, variation of inductance of the inductor ill and is readily adapted to provide any desired manner of variation of capacitance of the condenser l3. In addition, it has the important advantage that the inductor III' is eflectively enclosed within a shielding housing provided by the condenser i3, thus to confine th wave-signal fields of the parallel-resonant circuit l0, [3' within the condenser l3. Since a high figure of merit is desirable in a loop-antenna tuning system embodying the present invention, and since the value of the figure of merit varies directly with the Q of the inductor It, the condenser plates l9', 20 should have sufficient diameters that they do not unduly impair the Q of the inductor by their proximity thereto.

As illustrative of a specific embodiment of the invention, the following circuit constants are given for an embodiment of the invention of the type shown in Fig. 2:

Loop antenna 194 microhenries Etfectlve inductance of antenna 11 as coupled across the parallel-resonant circuit 10, 13 e 960 microhenries Inductor 10' 320 to 2860 microhenries (9 to 1 range of variation) Portion a 215 turns No. 7/44 SCE Litz wire progressive universal wound on coil form 0.335

inch 0. D. Bakelite; inductance=111 microhenries in a r, Q=71 Portion b 265 turns No. 7/44 SCE Litz wire progressive universal wound on coil form 0.223 inch 0. D. Bakelite; inductance= microhenries in a r,

Composite inductance of loop antenna 11 and inductor 10' in parallel-" 240 to 720 microhenries (3 to 1 range of variation) Condenser 13 40 to micromicrofarads (3 to 1 range of variation) Tuning range of antenna system 550 to 1650 kilocycles It should be noted in connection with these representative circuit constants that while the value of the loop-antenna inductance given is similar to that used with high-impedance condensertuned loop circuits, the loop antenna ii is decoupled from the circuit l0, I3 and thus has many of the advantages of low-impedance loop circuits.

From the foregoing description of the invention, it will be apparent that a loop-antenna tuning system embodying the invention has the advantage that it exhibits a substantially constant and high figure of merit with tuning of the system over a relatively wide range of operating frequencies. There is the additional advantage that th figure of merit obtainable may have a value substantially larger than heretofore readily obtainable where a constant figure of merit is of paramount importance.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A loop-antenna tuning system comprising: an inductor; a loop antenna effectively coupled across at least a portion of said inductor to provide therewith a composite value of inductance; a condenser, including a capacitance-adjusting element, coupled across said inductor to provide a parallel-resonant circuit having a predetermined Q at a frequency within a range of operating frequencies; and an inductance-adjusting element, associated with said inductor and mechanically coupled to said capacitanceadjusting element for movement in unison therewith to tune said circuit over said range, having a configuration and movement so related to the configuration and movement of said capacitance-adjusting element as to maintain over said range a ratio of composite inductance to capacitance for said circuit varying in opposite sense to any variations of Q thereof from said predetermined value of Q to maintain the figure of merit of said system substantially constant with said tuning of said circuit.

2. A loop-antenna tuning system comprising: an inductor; a loop antenna effectively coupled across at least a portion of said inductor to provide therewith a composite value of induct ance; a condenser, including a capacitance-adjusting element, coupled across said inductor to provide a parallel-resonant circuit having a predetermined Q at a frequency within a range of operating frequencies; and an inductance-adjusting element, associated with said inductor and mechanically coupled to said capacitance-adjusting element for movement in unison therewith to tune said circuit over said range, having a configuration and movement so related to the configuration and movement of said capacitance-adjusting element as to maintain over said range a ratio of composite inductance and aapacitance for said circuit varying approximately inversely proportional to the square of any variations of Q thereof from said predetermined value of Q to maintain the figure of merit of said system substantially constant with said tuning of said circuit.

3. A loopantenna tuning system comprising: an inductor; a loop antenna effectively coupled across at least a portion of said inductor to provide therewith a composite value of inductance; a condenser, including a capacitance-adjusting element, coupled across said inductor to provide a parallel-resonant circuit having a predetermined substantially constant value of Q at any frequency within a range of operating frequencies; and an inductance-adjusting element, associated with said inductor and mechanically cou pled to said capacitance-adjusting element for movement in unison therewith to tune said circuit over said range, having a configuration and movement so related to the configuration and movement of said capacitance-adjusting element as to maintain over said range a substantially constant ratio of composite inductance to capacitance for said circuit to maintain the figure of merit of said system substantially constant with said tuning of said circuit.

4. A loop-antenna tuning system comprising: an inductor; a loop antenna effectively coupled across at least a portion of said inductor to provide therewith a composite value of inductance; a condenser, including a capacitance-adjusting element, coupled across said inductor to provide a parallel-resonant circuit having a predetermined Q at a frequency within a range of operating frequencies; and a magnetic member, movably positioned in the magnetic field of said inductor and mechanically coupled to said capacitame-adjusting element for movement in unison therewith to tune said circuit over said range, having a configuration and movement so related to the configuration and movement of said capacitance-adjusting element as to maintain over said range a ratio of composite inductance to capacitance for said circuit varying in opposite sense to any variations of Q thereof from said predetermined value of Q to maintain the figure of merit of said system substantially constant with said tuning of said circuit.

5. A loop-antenna tuning system comprising: an inductor; a loop antenna effectively coupled across at least a portion of said inductor to provide therewith a composite value of inductance; a condenser, including a capacitance-adjusting element, coupled across said inductor to provide a parallel-resonant circuit having a predetermined Q at a frequency within a range of operating frequencies; and a powdered iron core, movably positioned in the magnetic field of said inductor and mechanically coupled to said capacitance-adjusting element for movement in unison therewith to tune said circuit over said range, having a configuration and movement so related to the configuration and movement of said capacitance-adjusting element as to maintain over said range a ratio of composite inductance to capacitance for said circuit varying in opposite sense to any variations of Q thereof from said predetermined value of Q to maintain he figure of merit of said system substantially con stant with said tuning of said circuit.

6. A loop-antenna tuning systemcomprising: an inductor having an effective length much greater than its diameter; a loop antenna effectively coupled across at least a portion of said inductor to provide therewith a composite value of inductance; a condenser, including a capacitance-adjusting element, coupled across said inductor to provide a parallel-resonant circuit having a predetermined Q at a frequency within a. range of operating frequencies; and a magnetic member, having a length comparable with that of said inductor, associated with said inductor and mechanically coupled to said capacitance-adjusting element for movement in unison therewith to tune said circuit over said range, and having a configuration and movement so related to the configuration and movement of said capacitanceadjusting element as to maintain over said range a ratio of composite inductance to capacitance for said circuit varying in opposite sense with any variations of Q thereof from said predetermined value of Q to maintain the figure of merit of said system substantially constant with said tuning of said circuit.

7. A loop-antenna tuning system comprising: an inductor having two portions wound over one another with aiding magnetic fields; a loop antenna connected across one of said portions of said inductor to provide with said inductor a composite value of inductance; said one portion of said inductor being selected to provide a substantial eiiective increase of said antenna inductance; a condenser, including a capacitanceadjusting element, coupled across said inductor to provide a parallel-resonant circuit having a predetermined Q at said frequency; and an inductance-adjusting element, associated with said inductor and mechanically coupled to said capacitame-adjusting element for movement in unison therewith to tune said circuit over said range,

14 having a configuration and movement so related to the configuration and movement of said capacitance-adjusting element as to maintain over said range a ratio of composite inductance to capacitance for said circuit varying in opposite sense to any variations of Q thereof from said predetermined value of Q to maintain the figure or merit of said system substantially constant with said tuning of said circuit.

BERNARD D. LOUGHLIN.

REFERENCES crrnn The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,244,177 Schaper June 3, 1941 

